how to avoid instance server overload

[Movix]

Hello everybody,
Sorry for this post to be a bit long.

I'm wondering how to handle the instance server load. In any case a constant monitoring is needed to make sure the server performance it good enough.
When having 30 days free test periods some test instances will be removed because customers to not switch to paid, but other hopefully will. This leads to an instance server open for test instances and paid instances.

I know about the SQL and shell script that can be configured when a test instance switches to a paid one giving the possibility to move the instance to a server dedicated for paid instances. But what about DNS propagation delays in this case ? Do i have to set my DNS TTL as low as possible to reduce the down time / period of unknown behaviors ?
This DNS propagation thing also kicks in when moving an instance for one instance server to another one for another reason, for example when one customer grows in a way the server can not anymore keep the overall performance at a satisfying level.

I'm thinking about a reverse proxy sitting in front of the instance servers leading to a stable DNS situation. But this would need heavy modification in the existing scripts for the various tasks in SellYouSaas and thus leading to break the update features of the SellYouSaas environnement.

Is there any advice how to build the SellYourSass infrastructure to properly handle this ?

best regards
Movix